The present invention relates to an apparatus designed to drive a liquid crystal element in an attempt to extend the operating temperature range of the element, particularly its low temperature region.
The apparatus of the present invention is adapted for driving a liquid crystal element operated within a wide temperature range as in an automotive glare prevention type reflex mirror or the like employing a DSM (dynamic scattering mode) liquid crystal.
The DSM liquid crystal element used heretofore generally has specific characteristics that its threshold voltage to generate a dynamic scattering mode varies depending on a temperature in such a manner as to become higher with temperature fall. Therefore, when the driving voltage is established at a low level for reducing power consumption of the liquid crystal element, its operating temperature range is rendered so narrow that a proper operation is impossible in a low temperature region. To the contrary, if the driving voltage is established at a higher level to widen the temperature range, the power consumption is increased with another disadvantage that an excess voltage far higher than the threshold voltage is applied under normal temperature condition, whereby the service life of the element is shortened.
For the purpose of eliminating the problems mentioned above, some improvements have already been contrived as disclosed in, for example, Japanese Patent Laid-open No. 50-93160 wherein a driving voltage for a liquid crystal element is increased with fall of the element temperature.
However, since the above apparatus is so formed that a relatively high input voltage is once decreased, prior to application to a liquid crystal element, in accordance with the resistance of a thermistor provided to sense the temperature of the element, there exist disadvantages that a considerably great voltage loss is unavoidable and, in case any variation occurs in the input voltage, the voltage applied to the liquid crystal element also varies to eventually fail in performing stable driving of the element.
As a result of the experiment conducted by the present inventors, it has been found that the temperature characteristic of the DSM liquid crystal element with respect to the threshold voltage thereof is such as graphically shown in FIG. 1 with the frequency used as a parameter. The DSM liquid crystal element employed in the experiment was composed of nematic liquid crystal DNM-4 (made by Dai Nippon Paint Co., Ltd.), and its substrate was processed with parallel orientation at an interval of 10 microns. When the element is driven by a voltage of a fixed frequency, as will be understood from the graphic representation, its threshold voltage increases in accordance with temperature fall. The graph further shows that the trend of increase of the threshold voltage induced with temperature fall appears to be more conspicuous with rise of the driving frequency and also that, in case the temperature is maintained constant, the threshold voltage becomes higher as the driving frequency rises.